The Cutting Edge
A Sound Proposition
Adding
Value to the Window System
by Jim Plavecsky
Thermal performance has been a hot topic over the last few years, but what else can be
done with the insulating glass (IG) unit to add value to the window system?
Sound Attenuation and Transmission
Just as homeowners rely upon the IG unit to keep heat either inside or outside of
the home, they also can benefit from a window that keeps unwanted sounds from creeping in
to disturb favorite indoor activities.
Sound is transmitted in one of three ways. It can be reflected, absorbed and converted to
another form of energy or it is transmitted through the object. When we seek to produce
quiet windows, we are attempting to achieve designs that reflect or absorb a greater
degree of sound energy while transmitting less.
Stiffness affects the transmission of low frequency sounds. Pop off the grille cover on
your stereo speakers and look at the woofer. This diaphragm easily moves in and out
reproducing the low-frequency part of the music program. Increasing the stiffness reduces
diaphragm action resulting in less sound transmission of lower frequency sounds.
Now, what about mid-frequency sounds? These are the sounds characterized by shorter
wavelengths and are transmitted transversely, like a flag fluttering in the wind. Every
material has a characteristic resonation frequency at which a standing wave builds upon
itself thereby increasing the amplitude of the wave. In designing windows we must choose
and combine materials to inhibit the formation of these waves.
Measuring Attenuation
Sound Transmission Class (STC) rating is calculated and used to evaluate the
impact that various design changes are making upon sound attenuation.
The STC rating, introduced in 1970 as ASTM E413, is a single number used to characterize
sound transmission loss data. It compares the contour curve generated in a test room
against a reference contour curve that represents an STC rating of zero. The number of the
contour that best fits the sound transmission data generated gives the STC rating for the
window being tested—the higher the STC rating, the better.
Because this method was the original procedure used, many building codes and government
regulations are based upon STC ratings. However, because it is based upon the types of
noise generated within office buildings, another method was needed to correlate more
closely with the types of noise generated outside. Therefore, the Outdoor Indoor
Transmission Class rating (OITC) was developed becoming ASTM E 1332. This method uses an
incident power spectrum that does a better job of also covering lower frequency sounds and
is more appropriate for evaluating attenuation of sounds generated by planes, trains and
automobiles. As a general rule of thumb, OITC ratings usually run about 5 to 10 decibels
lower than STC ratings.
Basic Design
There are quite a few factors to consider as we set out to design the quiet
window. A very important one is the mass-law relationship. It states that transmission
loss (TL) increases by 6 decibels every time we double the mass per unit area. In other
words—use thicker glass! Also, the use of insulating glass has a positive effect.
Doubling the air-gap will reduce sound transmission by 3 decibels. Gas-filling with Argon
also has a beneficial effect upon sound transmission loss.
Another design principle to keep in mind is the phenomenon of decoupling. Materials of
identical characteristics will respond to incident sound waves in similar ways. This is
referred to as coupling and it results in the formation of standing waves. Therefore, we
want to combine materials that are dissimilar. The most common examples here are using two
or more glass lites of different thickness or employing a film suspended between glass
lites.
Acoustic Shock Absorbers
Certain materials tend to absorb acoustical energy as opposed to transmitting it.
One example is the use of laminated glass, which also adds a safety feature to the window.
Another type of acoustic shock absorber is structural foam spacer. Although normally used
as a means of improving the thermal properties of the window system, structural foam
spacers have also been shown to improve the sound transmission loss characteristics of the
window system. Glazing components also play a part. The use of glazing materials with
sound dampening properties can also help reduce sound transmission through the edge of the
window unit.
When it comes to acoustical properties, less bang can turn into more bang for the buck!
Jim Plavecsky serves as vice president of marketing and sales for
Edgetech IG, based in Cambridge, Ohio.
DWM
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